Thermal management in an electronic system is a critical task to ensure the system's functionality as well as to maintain the reliability of its key components. Heat is mostly generated by active devices, such as logic and memory chips. Heat dissipation from an active device has been therefore an important design consideration, particularly due to the increase in the number of circuits per unit area. Thermal conduction mechanisms have been commonly employed to dissipate the heat from an active device by attachment of a heat sink or via a heat spreader attached to a heat sink. To attach a heat sink or a heat spreader to an active device, it is necessary to use a joining material, which should possess a good thermal conductivity among other properties. Otherwise, the joint between an active device and a heat sink becomes a bottleneck in terms of heat dissipation.
Silver-filled epoxy or other polymer resin materials as schematically shown in FIG. 1, are most commonly used as thermally conducting die-attach materials in microelectronics packages. See, for instance U.S. Pat. Nos. 5,391,604 (Dietz et al.), 5,399,907 (Nguyen et al.) 5,447,988 (Dershem et al.), 5,475,048 (Jamison et al.), 5,488,082 (Dietz et al.), and 5,504,374 (Oliver et al.).
Solder alloys are also used for thermally conducting joining materials. Silver-filled epoxy materials have several short-comings, such as low thermal conductivity, low joint strength, long curing cycle, etc.. Most solder alloys contain lead (Pb) as a major component which is not environment-friendly. Additionally, lead solder joints can form intermetallic compounds which, in turn, can lead to brittle interfaces that separate under thermal cycles.
In earlier patent applications, assigned to the same Assignee as is this application, the disclosures of which are incorporated herein by reference, electrically conductive paste materials are disclosed which can be used for thermal conduction applications:
Ser. No. 08/641,406; entitled "Electrically Conductive Paste Materials; Graham et al. PA1 Ser. No. 08/414,070; entitled "Dendritic Powder Materials for High Conductivity Paste Application; Kang et al. PA1 Ser. No. 08/868,771, entitled "Electrodeposition of Low Temperature, High Conductivity, Powder Materials from Electrically Conductive Paste Formulations"; Kang et al. PA1 Ser. No. 08/877,991; entitled "High-Conductivity, High Strength, Lead-Free, Low Cost, Electrically Conducting Materials and Applications"; Gelorme et al.
These electrically and thermally conductive paste materials comprise conducting filler particles dispersed in a matrix of thermoplastic polymer resin with other ingredients. They were developed as Pb-free, non-toxic and environment-friendly, assembly materials for electronic applications.
In one embodiment (Ser. No. 08/641,106), a new electrically conductive paste is disclosed made of tin-coated copper powder, polyimide-siloxane, solvent, carboxylic acid/surfactant, and no-clean flux. A joining operation can be performed with such a paste near the melting point of Sn, 230.degree. C., where a metallurgical bonding of Sn-to-Sn or Sn-to-Au or Cu is accomplished at the particle-to-particle as well as the particle-to-substrate pad interfaces. Because of the metallurgical bonding, higher electrical conductivity and higher joint strengths were demonstrated, as compared to joints made with silver-filled epoxy materials. The metallurgical bonds also provided stable electrical conductivity upon thermal exposure and cycling.
In co-pending application, filed on even date herewith and entitled "Method of Fabricating Coated Powder Materials and Their Use for High Conductivity Paste Applications" to Kang et al, Ser. No. 09/111,155, a new electrically conductive paste is disclosed made of bismuth-tin coated copper powder, thermoplastic polymer resin, solvent, and no-clean flux. A joining operation thereof can be performed near the eutectic temperature of the Bi-Sn alloy, which is about 139.degree. C.
In another embodiment (Ser. No. 08/868,7710), a new electrically conductive paste is disclosed made of indium-tin coated copper powder, thermoplastic polymer resin, solvent, and no-clean flux. A joining operation can be performed near the eutectic temperature of the In-Sn alloy, which is 120.degree. C.
It is an object of the present invention to provide a thermally conductive adhesive material which is environmentally safe and low cost.
It is another object of the present invention to provide a thermally conductive adhesive material which produces a higher thermal conductivity than does conventional silver-filled epoxy.
It is a further object of the present invention to provide a thermally conductive adhesive material which produces a higher joint strength than conventional silver-filled epoxy.
It is a still further object of the present invention to provide a method of fabricating a thermally conductive adhesive material which can be processed at a lower temperature than the reflow temperature of a Pb-Sn eutectic solder paste.
It is yet another object of the present invention to provide a thermally conductive adhesive material which produces a more reliable joint than conventional silver-filled epoxy.